In snowmobiles, transverse mass centralization is important in order to have good handling characteristics. If too much mass is placed to one side of the snowmobile, the snowmobile will tend to sink into the snow on that side making it difficult to steer. This is particularly true when the snowmobile is used in soft powdered snow.
Conventionally, snowmobiles have a pair of skis and an endless drive track for propelling the snowmobile, both of which are mounted to a frame of the snowmobile. The frame includes an engine compartment which conventionally has left and right sides, a bottom, a rear (bulkhead), and a front portion. The engine compartment supports power pack components such as an engine, a tuned pipe, a muffler, a reduction gearing, and a continuously variable transmission (CVT). The frame also includes a tunnel rearward of the engine compartment below which the endless drive track is disposed. It is common to have snowmobiles having a tunnel width of 34.3 cm (13.5 inches), 38.1 cm (15 inches), 50.8 cm (20 inches), or 61 cm (24 inches), depending on the intended use of the snowmobile. The CVT has a drive pulley mounted to an output shaft of the engine, a driven pulley, and a belt looped around the two pulleys. The driven pulley is mounted to a countershaft and has a  portion thereof which extends next to a side of the tunnel. The countershaft is operatively connected to the endless drive track so as to drive the endless drive track.
Since the engine is the component of the snowmobile which has the largest mass, it is important that the engine be centered as much as possible with respect to the longitudinal centerline of the snowmobile. However, this may not always be possible.
Since a portion of the driven pulley extends next to a side of the tunnel, as mentioned above, for two snowmobiles having different tunnel widths and identical power packs, the snowmobile having the larger tunnel width will need to have the engine more offset to one side of its longitudinal centerline than the other snowmobile, otherwise the driven pulley would interfere with the tunnel. This results in an unbalance in the transverse mass disposition. This unbalance can be counteracted by locating other components of the power pack on the other side of the longitudinal axis of the snowmobile. While this enables designers to design snowmobiles with different tunnel widths, it can be time consuming and costly since many parts of the power pack have to be relocated and/or redesigned each and every time a wider tunnel width is desired. Therefore each tunnel width requires its own power pack configuration. This can sometimes result in having to adopt different assembly procedures to accommodate the different power pack configurations which may increase production time and therefore the overall manufacturing cost of the snowmobiles.
Therefore, there is a need for a family of snowmobiles where the members of the family can be designed with different tunnels, having different tunnel widths, while reducing the changes that have to be made to the power pack configuration as compared with conventional snowmobiles.